Finally, if you wanted to, you could host a copy on your own CDN like Amazon's cloudshare. It is very cheap to host and if you are using it only for your sites, it might only cost you a few dollars a month.

Seeing as BootStrap is incredibly customisable, including via their site, wouldn't you be better off tailoring it (in the same way you might tailor the Modernizr JS packages) to more closely meet your requirements (such as colours and default font sizes, remove unused features, etc), and then host the (often much) smaller file your self?

Whilst there's some small benefit to not downloading the full payload if someone has already visited a site using it, there's still all the processing and parsing of the script that has to happen...